The present invention relates to opthalmic lenses and pertains particularly to a method of constructing lenses to have a high power and low weight.
In the past, glass has been the traditional lens material. Glass, however, has a number of disadvantages. One of the disadvantages being that it is difficult to fabricate into complex optical components. Another disadvantage is that they are very expensive to fabricate and, when used to make high-power lenses, are fairly heavy in weight. Still another disadvantage of glass lenses is that they are breakable. While glass lenses can be treated by tempering to reduce their fragile nature, over a period of time they lose their tough nature and can become scratched and pitted. Such lenses are then difficult to regrind to refinish the surface.
In recent years, plastic materials have come into use as a material for the production of optical components. They have the advantage of being less expensive to fabricate and are usually lighter in weight than an equivalent lens of the same optical size made of glass. The conventionally used plastics, however, have the drawback of having a lower index of refraction than glass, and for this reason an equivalent lens is thicker. This constitutes a disadvantage for the production of high-power lenses.
Another disadvantage of such plastic materials is that they are easily scratched. They also have the disadvantage of being easily distorted by mounting in frames when critical tolerances are not carefully observed.
Still another disadvantage of plastic is that it is hydrostatic and it also absorbs dirt.
There have been proposals in the past to overcome the dirt-absorbing and scratch problems of plastic lenses by incorporating them within, or covering them with, sheets of glass. Among the prior art approaches to these problems are the following patents: U.S. Pat. No. 2,287,546, issued June 23, 1942 to Binda; U.S. Pat. No. 2,361,589, issued Oct. 31, 1944 to Bennett et al; and U.S. Pat. No. 2,540,953, issued Feb. 6, 1951 to Kessler.
These, however, have not considered the problem of reducing the weight and thickness of a high-power, low-cost lens.